The present invention relates to an engine exhaust purification arrangement provided with a catalyst and more specifically to a control arrangement for an exhaust gas purification arrangement which maintains the air-fuel ratio in a catalytic converter at stoichiometric using specific oxygen adsorption/release characteristics of the catalyst.
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein the amount of oxygen stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is computed based on an engine intake air amount and an air-fuel ratio of an exhaust flowing into the catalyst, and wherein engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is maintained essentially constant.
In order to maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of a three-way catalyst at a maximum or optimal level, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. By maintaining the oxygen storage amount of the catalyst constant, oxygen in the exhaust is stored in the catalyst when the air-fuel ratio of the exhaust flowing into the catalyst shifts to lean, and oxygen stored in the catalyst is released when the air-fuel ratio of the exhaust flowing into the catalyst shifts to rich, so the catalyst atmosphere can be assuredly maintained at the stoichiometric air-fuel ratio.
In an exhaust purification arrangement which performs this control, the conversion efficiency of the catalyst depends on the oxygen storage amount of the catalyst. Therefore, to control the oxygen storage amount to the desired constant level and to maintain the conversion efficiency of the catalyst at a high/optimal level, the oxygen storage amount must be precisely computed.
However, this system suffers from the drawback that it has proven difficult to precisely compute the oxygen storage amount of the catalyst using computational methods which have thus far been developed.
It is believed that this drawback results from the fact that the oxygen is stored and released rapidly by noble metal (platinum Pt, for example) contained in the catalyst, while stored and released slowly by an oxygen storage material such as cerium oxide, and that the amount of oxygen which is stored has not, in the prior art, been computed in a manner which takes this factor into account.
Therefore, as one approach to solving this problem, it is proposed to improve the precision with which the oxygen storage amount is determined by separately computing a high speed component/amount and a low speed component/amount and thus more accurately match the actual adsorption/release characteristics. Additionally, it is proposed to maintain the catalyst atmosphere based primarily on the high speed component and in a manner which maintains the catalyst conversion efficiency at a high-level. This is achieved by controlling the air-fuel ratio of the engine so that the high speed component is kept essentially constant at a target level.
However, if the air-fuel ratio of the exhaust gases, which are emitted from the engine and supplied to the catalyst, is strongly shifted into the lean region when the storage amount of the high speed component is effectively zero, or the engine air-fuel ratio is largely varied to rich when the storage amount of the high speed component has reached its maximum capacity, i.e., if the air-fuel ratio is varied without any restriction to maintain the high speed component constant, drivability and fuel economy decrease. On the other hand, when the oxygen storage capacity of the catalyst is a maximum, the NOx discharge amount tends to increase, and at such times, the oxygen storage amount must be rapidly decreased to the optimum amount.
This invention is therefore directed to controlling the amount of oxygen stored in a catalyst to an appropriate amount with good response so that the air-fuel ratio can be controlled to the degree that the oxygen storage amount of the catalyst is essentially constant while obviating the decrease of drivability and fuel-cost performance due to excessively large variations in the air-fuel ratio.
In order to achieve the above, a first aspect of the invention provides an exhaust purification arrangement for an engine which includes a catalyst provided in an exhaust passage of the engine, a front sensor for sensing the air fuel ratio of the exhaust gases which are flowing to the catalyst and a processor (e.g. microprocessor) arrangement which is responsive to the detected air/fuel ratio. In this arrangement the catalyst contains a material (or materials) which stores oxygen in the form of a high speed component wherein the oxygen is rapidly adsorbed and released from the material and which stores oxygen in the form of a low speed component wherein the oxygen is slowly adsorbed and released from the material. The microprocessor is programmed to compute the high speed oxygen storage amount of the material based on the detected exhaust air-fuel ratio, and to compute the target air-fuel ratio to be supplied to the engine (1) so that the amount of oxygen in the exhaust gas maintains the high speed oxygen storage amount in the material at a predetermined target value which is selected to be able to absorb or releases oxygen as required during transient periods and enables the atmosphere about the catalyst to be maintained at an air-fuel ratio which promotes efficient conversion of noxious compounds.
Another aspect of the invention resides in a method of controlling the air-fuel ratio of an atmosphere in an catalytic converter which is operatively connected with an internal combustion engine, comprising the steps of: storing oxygen on a first material in the catalytic converter which adsorbs and releases oxygen rapidly; storing oxygen on a second material in the catalytic converter which adsorbs and releases oxygen more slowly than the first material; and controlling the air-fuel ratio of the exhaust gas entering the catalytic converter to control the amount of oxygen which is adsorbed on the first material to a predetermined target amount which is less than the maximum amount of oxygen which can be adsorbed onto the first material.
In this method the step of controlling the air-fuel ratio of the exhaust gases can include controlling the air-fuel ratio of the exhaust gases to within upper and lower air-fuel ratio limits; determining if the first material is saturated with oxygen; and temporarily reducing the lower air-fuel ratio limit to enrich the air-fuel mixture to rapidly lower the amount of oxygen stored in the first material toward the target value which, as mentioned above, is selected to be between maximum and minimum amounts of oxygen which can be stored by the first material and thus allow oxygen to be rapidly stored or released from the first material in a manner which enables quick response to fluctuations in the air-fuel ratio of the incoming gases.
A further aspect of the invention resides in an arrangement for controlling the air-fuel ratio of an atmosphere in an catalytic converter which is operatively connected with an internal combustion engine, comprising: a first material disposed in the catalytic converter which adsorbs and releases oxygen rapidly; a second material disposed in the catalytic converter which adsorbs and releases oxygen more slowly than the first material; a control arrangement for controlling the air-fuel ratio of the exhaust gas entering the catalytic converter to adjust the amount of oxygen which is adsorbed on the first material to a target amount which is approximately half of the maximum amount of oxygen which can be adsorbed onto the first material.
The control arrangement in this case is also capable of executing the steps set forth above, so that should the first material become saturated with oxygen due to a temporary leaning of the air-fuel ratio such as results from no fuel being supplied to selected cylinders of the engine during so called xe2x80x9cfuel cutxe2x80x9d modes of operation, and additionally enriched air fuel mixture can be supplied to rapidly reduce the amount of oxygen stored on the first material toward the target amount.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.
In more detail, noble metals adsorb oxygen in the molecular state, while oxygen storage materials absorb oxygen as compounds, but in the following description, adsorption and absorption will be collectively referred to as storage. Further, the expression xe2x80x9cthe exhaust air-fuel ratio is richxe2x80x9d will be used throughout the specification to refer to the situation wherein the oxygen concentration in the exhaust is lower than the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio, while the expression xe2x80x9cthe exhaust air-fuel ratio is leanxe2x80x9d will be used to denote the situation wherein the oxygen concentration in the exhaust is higher than the oxygen concentration when the engine is running at the stoichiometric air-fuel ratio. The expression xe2x80x9cthe exhaust air-fuel ratio is stoichiometricxe2x80x9d will be used to indicate that the oxygen concentration in the exhaust is equal to the oxygen concentration of the exhaust when the engine is running at the stoichiometric air-fuel ratio.